The present invention relates generally to fiber optic devices. Specifically, the present invention relates to an all fiber attenuator.
Although a primary virtue of optical fibers as communication media is the low loss of the fibers, there are instances where it is necessary to provide attenuation in the optical path. A variety of methods have been disclosed to fabricate optical fiber attenuators.
In U.S. Pat. No. 4,639,078 the end of a fiber is coated with a thin layer of an adhesive liquid containing submicron light absorbing particles. The liquid is spread evenly over the end of the fiber before it is cured. After solidification the coated end of the fiber is spliced to the end of a second fiber.
In U.S. Pat. No. 5,095,519 a section of coreless, undoped fiber is fusion spliced into an optical fiber transmission path, both fibers being of the same diameter. Upon encountering the insert the beam diameter expands to a pre-selected diameter at the end of the insert. The ratio of the beam diameter at the end of the insert to the diameter of the adjacent fiber end determines the degree of attenuation.
In U.S. Pat. No. 4,884,859 an optical attenuation segment within a fiber is created by forming fine cracks in the attenuating segment. The cracks are formed by heating a segment of the fiber and simultaneously applying tension to that segment.
In U.S. Pat. No. 4,529,262 a birefringent polarization-preserving fiber and a single polarization fiber are combined. The polarization-preserving fiber acts as a variable wave plate and the single-polarization fiber acts as a polarizer. The degree of attenuation can be modified by exposing the fiber to tension, pressure, or temperature, thereby altering the birefringence in the polarization-preserving fiber.
In U.S. Pat. No. 4,881,793 an attenuating fiber segment is spliced into a signal carrying fiber. The attenuating segment is formed by vapor doping a length of fiber and then cutting the fiber to an appropriate length to achieve the desired level of attenuation.
The above attenuators suffer from a number of performance disadvantages. For example, in the '859 patent the number and size of the cracks regulate the degree of attenuation, two factors which are difficult to precisely control. A second problem can be the environmental stability of the attenuator. In particular, the cracks in the '859 fibers and the polarization of the '262 polarization-preserving fibers are environmentally sensitive, thus increasing the costs associated with these attenuators as well as limiting their possible applications. Wavelength and polarization sensitivities of some of attenuators adds even further limitations to their use.
From the foregoing, it is apparent that an optical attenuator is desired which may be inexpensively produced and which may provide a precise attenuation of light energy over a broad range, and for different configurations.